Organic semiconductors have received a great deal of attention in recent years owing to their low costs, the possibility of tailoring them to suit large areas and flexible substrates and the vast selection of corresponding molecules. Organic semiconductors can be installed in switchable components such as transistors and also in optoelectronic components such as organic light-emitting diodes (OLEDs) and photovoltaic cells.
Organic transistors, in particular organic field-effect transistors (OTFT), have been investigated and developed for many years now. It is anticipated that a large number of OTFTs can be used for example in inexpensive integrated circuits for non-contact identification tags (RFID) but also for screen control (Backplane). In order to achieve inexpensive applications, generally thin-layer processes are required to manufacture the transistors. In recent years performance features have been improved to such an extent that the commercialization of organic transistors is foreseeable. For example, in OTFTs high field-effect mobilities of up to 6 cm2/Vs for electrons on the basis of fullerene C60 and up to 5.5 cm2/Vs for holes on the basis of pentacene (Lee et al., Appl. Lett. 88, 162109 (2006)) have been reported.
Charge carrier transport in thin organic layers is generally described by temperature-activating charge carrier hopping which leads to relatively low mobilities and a strong influence of disorder. Therefore the field-effect mobility in OTFTs generally depends upon the charge carrier density (Vissenberg et al., Phys. Rev. B 57, 12 964 (1998)), Horowitz et al., Phys. Rev. B 66, 195336 (2002)). A relatively high gate voltage is generally necessary in order to fill localized states which do not contribute to electric transport and in order to achieve a high level of charge carrier mobility in the organic layer.
One option for increasing the charge carrier density and as a consequence also the charge carrier mobility in organic semiconductors is electrical doping using donors or acceptors. In so doing, by creating charge carriers in a matrix material, the Fermi level in the semiconductor is changed and also, depending upon the type of donors used, the initially quite low level of conductivity is increased. Document U.S. Pat. No. 5,093,698 describes general requirements placed on combinations of organic materials for electrical doping.
Electrical doping of organic semiconductors using molecular dopants has been investigated in detail over recent years. These investigations have shown that the mobility of mixed-layers increases depending upon the doping concentration. This phenomenon is explained by the fact that additional charge carriers gradually fill states of the matrix material from the lower distribution end of the density of states, i.e. states with low mobility (Maenning et al., Phys. Rev. B 64 195208 (2001)). In so doing, the Fermi level of the semiconductor is gradually changed to the same extent depending upon the type of donor used. It is increased in the case of n-doping. It is reduced for p-doping. This also increases the initially quite low level of conductivity.
In OTFTs which have an electrically doped active layer, the increased mobility also reduces the threshold voltage and thus also in general the operating voltage. For most areas of application for OTFTs it is desirable to achieve extremely low Off-currents. A high doping concentration creates a high background charge density which in turn leads to an undesired ohmic charge carrier transport which cannot be effectively controlled by the field effect.
The term ‘Off-state of a transistor’ is understood here to be an applied gate voltage smaller than the threshold voltage of the component for n-type conductors and greater than the threshold voltage for p-type conductors. In the case of the generally discussed OTFT in the enhancement mode the Off-state is present with the gate voltage Vg=0 V for p- and n-type.
However, it was found that in semiconductor layers with excellent charge carrier mobility the addition of donors increased the impurity scattering and thus also limited the maximum mobility in OTFT. (Harada et al., Appl. Phys. Lett. 91 092118 (2007)). An alternative arrangement is therefore desirable, where the charge carrier background concentration is increased without mixing donors in the semiconductor layer. In principle, such an arrangement allows to increase the charge carrier mobility above the usual amount.
The properties of the various materials used during an electric doping process can also be described by the energy layers of the lowest unoccupied molecular orbital (LUMO, synonym: ionization potential) and of the highest occupied molecular orbital (HOMO, synonym: electron affinity).
Ultraviolet photoelectron spectroscopy (UPS) is one method of determining the ionization potentials (IP). Generally, ionization potentials are determined for the solid state body, however, it also possible to measure the ionization potentials in the gas phase. Both parameters differ as a result of the solid state body effects such as, for example, the polarization energy of the holes which occur during the photo-ionization process. A typical value for the polarization energy is approx. 1 eV, but greater deviations therefrom can also occur. The ionization potential relates to the beginning of the photo-emission spectrum in the range of the high kinetic energies of photoelectrons, i.e. the energy of the weakest bound photoelectrons. Inverted photo electron spectroscopy (IPES) which is one method associated with this can be used to determine electron affinities (EA). However, this method is not widely used. Alternatively, solid state body energy levels can be determined by electrochemical measurements of oxidation potentials Eox or reduction potentials Ered in solution. One suitable method is for example cyclic voltammetry.
There are no known empirical formulae for converting reduction potentials into electron affinities. This is because of the difficulty in determining the electron affinities. A simple rule is therefore frequently used: IP=4.8 eV+e*Eox (see Ferrocene/Ferrocenium) or EA=4.8 eV+e*Ered (in comparison to Ferrocene/Ferrocenium) (cf. Andrade, Org. Electron. 6, 11 (2005)). In the event that other reference electrodes or redox pairs are used to reference electrochemical potentials, there are known methods for the conversion.
It is usual to use the terms “Energy of HOMOs” E(HOMO) or “Energy of the LUMOs) E(LUMO) synonym with the terms ionization energy or electron affinity (Koopmans Theorem). It is to be noted that the ionization potentials and electron affinities are such that a higher value represents a stronger bond of a removed or rather attached electron. Therefore the global approximation: IP=−E(HOMO) and EA=−D(LUMO) applies.
OTFTs with arrangements of additional layers on the active semiconductor layer, which additional layers are also designated as encapsulation or cover layer, have been described. For example, double layers of pentacene and C60 are used to achieve ambipolar component functionality (Wang et al., Org. Electron. 7,457 (2006)). In this special case, it can be derived from the energy levels that there has been no technically relevant change in the charge carrier density in the active layer. Document US 2007/034860 also describes such a structure and even claimed a higher mobility for the active layer in comparison to the encapsulation layer.